This is a study of regulation of the activity of the Lck tyrosine protein kinase. Lck plays a critical role in T cell development in the thymus and in T cell activation in mature T cells. In addition, Lck is the target of the product of the Tip oncogene of herpesvirus saimiri. This simian herpesvirus induces an aggressive T cell leukemia. Tip, the product of one of its oncogenes, binds to Lck and activates it. Given the important role of Lck in the regulation of T cell proliferation, it is very likely that the targeting of Lck by the viral Tip protein plays an important role in the induction of T cell malignancy. Three projects will be pursued. First, the structure of Tip will be determined using X-Ray crystallography. This should provide important insights into how it binds to Lck and how this binding activates the kinase. Second, the role of the tyrosine phosphorylation of Tip in the activation of Lck will be examined by mutagenesis, peptide mapping and characterization of the properties of forms of Tip lacking specific sites of phosphorylation. If this work suggests that the phosphorylation of Tip induces the binding of cellular signaling proteins, such binding partners will be sought. Additionally, a study of the biosynthesis of Tip will be carried out. Questions that will be addressed include how Tip is inserted into cellular membranes, where and when it binds to Lck, and whether oligomerization of Tip is important for the activation of Lck. Although it is possible that the mechanism by which Tip activates Lck is unique to virally-infected cells, it is more likely that Tip is exploiting or intervening in a regulatory mechanism that controls the activity of Lck in normal T cells. The third project will be a search for cellular Tip-like proteins using a yeast two-hybrid screen. The possibilities that the abundance of regulators of this class will be tightly controlled, or that their ability to bind to Lck will be modulated by phosphorylation, will be examined.